An epidemiological study of genetic markers in a sample of 111 pairs of twins and their parents. In this sample an estimate is available of the timing of conception in relation to ovulation, calculated from a knowledge of the characteristics of the menstrual cycle and the date of resumption of intercourse following the menstrual Jewish ritual sexual separation. Of the twins, 43 were born from conceptions occurring "late" in relation to the estimated day of ovulation and, therefore, probably from overripe ova. The remaining twins appeared to have been conceived normally. The objective is to determine why twinning was so markedly increased in the "late" fertilizations and to test the hypothesis that this is due to a third type of twinning, uniovular dispermatic, following the premature division of the ovum prior to fertilization. The investigators will test blood groups, HLA phenotypes, isoenzymes, nucleolar organizing regions (NORs) and chromosome C- and Q-bands. They will detect uniovular dispermatic twinning if the twins are concordant for all alleles received from the mother, while partially discordant for those received from the father. They will also develop mathematical models for estimating the probability of detecting first and second polar body twins and for distinguishing them from dizygous sets. They may also detect polar body twinning. Existence of a third type of twinning would necessitate a reappraisal of the theoretical and mathematical basis for heritability studies in genetic epidemiology. Although geneticists have long postulated the existence of uniovular dispermatic and polar body twins, and have suggested that they are more likely to occur after fertilization of overripe ova, no systematic search has ever been made for them before.